Eeg studies were carried out on the developing 1-day-old to 4-month-old chick under unrestrained conditions. Electrodes were chronically implanted in the rostral and the caudal parts of the frontal bone. Fast waves of 17 to 25 cps and 30 to 60 cps with a low voltage of 25[mu]V appeared during the opening of the eyes in the 1-day-old chick. Very slow waves of 1.5 to 5 cps with high amplitude were predominant during the drowsy or the sleeping stage until 1-week of age. Until 1-month of age, high amplitude and slow waves of 6 to 12 cps appeared during drowsy or sleeping state. Two month-old birds showed adult characteristics in both behavior and the eeg patterns. Furthermore, during paradoxical sleep, high amplitude and slow waves were often interrupted with low voltage and fast waves similar to those of the wakened state. This episode was observed in 1-day-old chicks, and lasted for 2 to 11 sec., mainly in the 3 to 4 sec. range. In addition, arousal eeg patterns associated with opening eyes were observed in the contralateral hemisphere of the forebrain in developing chicks from 1-day to 2-months of age. The arousal response to auditory stimuli in the somnolent state existed in the 1-day-old chick, when the bird was awakened by sound. During the so-called "paradoxical" sleep, the low voltage and fast waves were shifted to high amplitude and slow waves on the eeg by application of a mild or weak brief auditory stimulus, though no significant changes in sleeping behavior occurred. In developing chicks, the hypnotic or cataleptic state was readily induced by abnormal enforced positions or the act of being hand-held. The younger the birds, the more readily they succumbed into the hypnotic state represented by high amplitude and slow waves which is characteristic of the drowsy or sleeping state. Various types of seizure-like discharges were sometimes found under the drowsy or sleeping state in the developing chick, particularly in the newly hatched state. The persistence of those discharges varied, ranging from a little less than a minute to several minutes. These occurred locally or unilaterally in the rostral and the caudal parts of forebrain explored. The seizure-like discharges were induced also by intraper-itoneal injection of large doses of Nembutal in newly hatched birds.